In June 1984 biologists discovered a small breeding population of less than 10 Sheath-tailed Bats (Emballonura semicaudata) in two limestone caves on Goat Island (Aguijan) in the U.S. Commonwealth of the Northern Mariana Islands (CNMI). This discovery, and follow-up observations in 1985, represents the first verifiable record of this species from the Mariana archipelago in over 50 years. During a 1983-85 survey, biologists from the newly formed CNMI Division of Wildlife also contacted people who reported seeing small "cave bats" within the last five years on three other widely separated islands in the CNMI. Each sighting was of only a few individuals.
By popular account, Sheath-tailed Bats were once abundant. Colonies remained on some Mariana islands in the 1940's and 50's, but by the 1970's they were almost completely gone. The status of Sheath-tailed Bats was recorded in 1976 by the Guam Division of Aquatic and Wildlife Resources as "Last known sight record in May, 1972. Believed to be extinct." In 1981, the Governor of Guam petitioned the U.S. Fish and Wildlife Service to list the Mariana population of Sheath-tailed Bats as a federally endangered species. Two years later the request was denied based on lack of information about the bat's existence.
Sheath-tailed Bats are small insectivorous bats weighing only about five grams and with a wingspan of nine inches. They live in caves on the Pacific Islands of Samoa, Fiji, the New Hebrides, Palau and the Marianas. The species was last collected from Guam in 1895 and from the Northern Marianas in 1932. The Marianas specimen, from the island of Rota, was described as a new subspecies, but it has disappeared and may be extinct.
As with other poorly studied species, the exact reason for their population decline is unknown. Possible causes include the extensive impact of World War II on the Marianas. Heavy bombing and shelling, especially of fortified caves, occurred on some islands. Following the war the Marianas also received heavy doses of insecticides for public health and agricultural purposes, likely affecting the bats through the food chain. In addition, the mining of both Vanikoro Swiftlet (Aerodramus vanikorensis) and bat guano from limestone caves may have caused disturbances that neither species could tolerate. The birds inhabit the same caves as bats, and their populations also have declined since WWII.
Now that a breeding population of Sheath-tailed Bats has been located, measures to protect and enhance them can be undertaken. The Goat Island population should be carefully monitored while other islands are periodically re-surveyed. Receiving federally protected status would focus local and scientific attention on this rare native species. Limestone cave habitats must be actively protected to serve the best interests of bats as well as the also endangered swiftlet. Although the future is not bright for Sheath-tailed Bats in the Marianas, the epitaph of "Believed to be extinct" has been forestalled for the moment.
Dr. Thomas Lemke is a biologist with the Montana Dept. of Fish, Wildlife and Parks. For the past ten years he has studied the behavior and ecology of bats in South America and in the Pacific Islands. Dr. Lemke's article on the plight of the Marianas Fruit Bat appeared in the March 1986 issue of BATS.